Method and system for generating logarithmic non-uniform pseudo-random electromagnetic exploration signal

Abstract

A method and system for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal including: constructing two or more basic unit signals, according to an exploration requirement, that are stairstep signals obtained by superposing in-phase periodic square wave signals, a frequency ratio between adjacent periodic square wave signals is 2, and the two or more basic unit signals meet the following requirement: if a lowest dominant frequency in a first basic unit signal is a fundamental frequency, lowest frequencies of the remaining basic unit signal are l×2 m times the fundamental frequency, where l is an odd number except 1, and m is a natural number; and superposing the two or more basic unit signals to obtain a logarithmic non-uniform 2 n sequence pseudo-random signal. A logarithmic non-uniform 2 n sequence stairstep signal is constructed within a limited frequency interval, and requirements of prospectors for a higher frequency density within a specific frequency interval are met.

What is claimed is: 
     
       1. A method for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal, the method comprising:
 constructing two or more basic unit signals according to an exploration requirement, wherein 
 the basic unit signals are stairstep signals obtained by superposing a plurality of in-phase periodic square wave signals, a frequency ratio between adjacent ones of the plurality of periodic square wave signals is 2, and the two or more basic unit signals meet the following requirements: if a lowest frequency of dominant frequencies in a first basic unit signal is denoted as a fundamental frequency, lowest frequencies of the remaining basic unit signal are l×2 m  times the fundamental frequency, and the lowest frequencies of the basic unit signals differ from each other, wherein l is an odd number except 1, and m is a natural number; and 
 superposing the two or more basic unit signals to obtain superposed stairstep signals, and correcting amplitudes to be consistent with amplitudes of the periodic square wave signals, to obtain a logarithmic non-uniform 2 n  sequence pseudo-random signal; 
 wherein, the logarithmic non-uniform 2 n  sequence pseudo-random signal comprises different frequency densities in different frequency intervals, by adjusting highest and lowest frequencies of the basic unit signals and controlling of a multiple between the lowest frequencies of dominant frequencies in the basic unit signals to be l×2 m ; 
 performing a geophysical field exploration by using the logarithmic non-uniform 2 n  sequence pseudo-random signal, continuously receiving electromagnetic information and field data focused in frequency intervals of interest to prospectors by receivers in a rolling manner; wherein, the logarithmic non-uniform 2 n  sequence pseudo-random signal is able to be simultaneously transmitted and received at dozens of frequencies; and 
 determining at least one physical characteristic of an underground subject to the geophysical field exploration by analyzing the logarithmic non-uniform 2 n  sequence pseudo-random signal. 
 
     
     
       2. The method for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal according to  claim 1 , wherein the two or more basic unit signals meet the following requirements:
 a sum of quantities of dominant frequencies of all of the basic unit signals is an odd number; and 
 a quantity of the dominant frequencies of the first basic unit signal is greater than those of other basic unit signals, lowest frequencies and highest frequencies of the other basic unit signals fall within a frequency interval between a lowest frequency and a highest frequency of the first basic unit signal, and the basic unit signals have no repeated frequencies. 
 
     
     
       3. The method for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal according to  claim 1 , wherein a method for generating the periodic square wave signal comprises:
 generating sinusoidal signals having the same frequency as a target periodic square wave, and setting values of the sinusoidal signals that are greater than 0 to A and values of the sinusoidal signals that are less than 0 to −A, to obtain a series of in-phase periodic square wave signals, wherein A≠0; and 
 assigning A or −A to positions of periodic square wave signals having a value of 0 to obtain periodic square wave signals having no value of 0. 
 
     
     
       4. The method for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal according to  claim 3 , wherein the step of assigning A or −A to positions of the periodic square wave signals having a value of 0 specifically comprises:
 identifying positions of the periodic square wave signals having a value of 0 by using an index, setting odd positions to A, and setting even positions to −A. 
 
     
     
       5. The method for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal according to  claim 1 , the method further comprising:
 performing a phase adjustment on at least one of the two or more basic unit signals, and finding, as an optimal phase of the corresponding basic unit signal, a phase that minimizes a relative mean square (RMS) error of spectrum values corresponding to dominant frequencies in a superposed logarithmic non-uniform 2 n  sequence pseudo-random signal; and 
 superposing the two or more basic unit signals according to the optimal phase to obtain a final logarithmic non-uniform 2 n  sequence pseudo-random signal. 
 
     
     
       6. The method for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal according to  claim 5 , wherein a method for finding the optimal phase comprises:
 for the basic unit signals on which the phase adjustment is performed, dividing a phase radian π in half a period into N parts, to obtain phase-changed basic unit signal π/N; 
 adjusting one phase-changed basic unit signal each time, and recording the RMS error of spectrum values corresponding to dominant frequencies in the superposed logarithmic non-uniform 2 n  sequence pseudo-random signal; and 
 drawing a variation curve of the RMS error with a phase, and finding a phase, as the optimal phase of the basic unit signal, that minimizes the RMS error. 
 
     
     
       7. An electromagnetic exploration system, comprising an electrical exploration signal transmitter, a terminal device, and a plurality of receivers; wherein,
 the electrical exploration signal transmitter, being configured to transmit a logarithmic non-uniform 2 n  sequence pseudo-random signal generated by using a method for generating a logarithmic non-uniform pseudo-random electromagnetic exploration signal to a filed; wherein, the logarithmic non-uniform 2 n  sequence pseudo-random signal is able to be simultaneously transmitted and received at dozens of frequencies; 
 the plurality of receivers, being configured to continuously receive the logarithmic non-uniform 2 n  sequence pseudo-random signal with field data on a rolling manner; and 
 the terminal device, comprising a processor and a non-transitory computer-readable storage medium, wherein, the non-transitory computer-readable storage medium is configured to store a plurality of instructions, wherein the instructions are adapted to be loaded by the processor to perform the method for generating the logarithmic non-uniform pseudo-random electromagnetic exploration signal; and 
 when the processor implements the instructions, executing: 
 construct two or more basic unit signals according to a geophysical field exploration requirement, wherein the basic unit signals are stairstep signals obtained by superposing a plurality of in-phase periodic square wave signals, a frequency ratio between adjacent ones of the plurality of periodic square wave signals is 2, and the two or more basic unit signals meet the following requirement: if a lowest frequency of dominant frequencies in a first basic unit signal is denoted as a fundamental frequency, lowest frequencies of the remaining basic unit signal are l×2 m  times the fundamental frequency, wherein l is an odd number except 1, and m is a natural number; 
 obtain the logarithmic non-uniform 2 n  sequence pseudo-random signal by superposing the two or more basic unit signals; and 
 determine at least one physical characteristic of an underground subject to the geophysical field exploration by analyzing the logarithmic non-uniform 2 n  sequence pseudo-random signal. 
 
     
     
       8. The electromagnetic exploration system according to  claim 7 , wherein the two or more basic unit signals meet the following requirements:
 a sum of quantities of dominant frequencies of all of the basic unit signals is an odd number; and 
 a quantity of the dominant frequencies of the first basic unit signal is greater than those of other basic unit signals, lowest frequencies and highest frequencies of the other basic unit signals fall within a frequency interval between a lowest frequency and a highest frequency of the first basic unit signal, and the basic unit signals have no repeated frequencies. 
 
     
     
       9. The electromagnetic exploration system according to  claim 7 , wherein a method for generating the periodic square wave signal comprises:
 generating sinusoidal signals having the same frequency as a target periodic square wave, and setting values of the sinusoidal signals that are greater than 0 to A and values of the sinusoidal signals that are less than 0 to −A, to obtain a series of in-phase periodic square wave signals, wherein A≠0; and 
 assigning A or −A to positions of periodic square wave signals having a value of 0 to obtain periodic square wave signals having no value of 0. 
 
     
     
       10. The electromagnetic exploration system according to  claim 9 , wherein the step of assigning A or −A to positions of the periodic square wave signals having a value of 0 specifically comprises:
 identifying positions of the periodic square wave signals having a value of 0 by using an index, setting odd positions to A, and setting even positions to −A. 
 
     
     
       11. The electromagnetic exploration system according to  claim 7 , further comprising:
 performing a phase adjustment on at least one of the two or more basic unit signals, and finding, as an optimal phase of the corresponding basic unit signal, a phase that minimizes a relative mean square (RMS) error of spectrum values corresponding to dominant frequencies in a superposed logarithmic non-uniform 2 n  sequence pseudo-random signal; and 
 superposing the two or more basic unit signals according to the optimal phase to obtain a final logarithmic non-uniform 2 n  sequence pseudo-random signal. 
 
     
     
       12. The electromagnetic exploration system according to  claim 11 , wherein a method for finding the optimal phase comprises:
 for the basic unit signals on which the phase adjustment is performed, dividing a phase radian π in half a period into N parts, to obtain phase-changed basic unit signal π/N; 
 adjusting one phase-changed basic unit signal each time, and recording the RMS error of spectrum values corresponding to dominant frequencies in the superposed logarithmic non-uniform 2 n  sequence pseudo-random signal; and 
 drawing a variation curve of the RMS error with a phase, and finding a phase, as the optimal phase of the basic unit signal, that minimizes the RMS error.